Microelectrophoresis techniques have been used in the female rat anesthetized with urethane to identify the tuberoinfundibular neurons and to determine their pharmacological sensitivity to monoamines. Arcuate neurons whose axons terminate in the median eminence as determined by antidromic identification have been shown to form at least two distinct populations: Those arcuate neurons which are excited by norepinephrine and those which are excited by dopamine. The results are compatible with a functional role for both dopamine and norepinephrine in modulating pituitary function by altering the activities of presumed neurosecretory neurons which release hypophysiotropic hormones.